In the past decades, it has already often been proposed to improve the properties of polymeric materials by including in these materials a specific amount of a clay. The presence of a clay in a polymeric material particularly contributes to properties such as the mechanical strength and the heat resistance of the polymeric material.
A great difficulty when including a clay in a polymeric matrix resides in the mutually rather different nature of the materials. The polymeric material of the matrix is a usually non-polar organic material, whereas the clay is a much more polar inorganic material. Because of this difference the materials are poorly intermixable; they are intrinsically not mixable.
To circumvent this difficulty, it has been proposed to carry out the synthesis of the polymer that forms the matrix, a polymerization reaction, in the presence of the inorganic material. The idea was that the clay is perhaps more easily intermixable with a monomeric material than with a polymeric material. This method, however, proved to result in an inhomogeneous product which does not have the desired properties. The clay has a layered or fibrous structure which cannot be broken easily, so that a homogeneous mixing is hard to obtain.
U.S. Pat. Nos. 4,889,885 and 4,810,734 propose to first swell the clay before adding monomeric material and carrying out a polymerization. To this end, the clay is modified with a swelling agent which increases the mutual distance between the layers of the clay structure to such an extent that monomers fit there between. After polymerization of the monomers polymeric material is then automatically located between the clay layers.
The swelling agent described in the above patents is so-called onium ions. Within this context an onium ion is a surfactant with a head group formed by an ammonium, pyridinium, sulfonium or phosphonium group, and one or more non-polar tails. The cationic head group of an onium ion is exchanged for cations between the crystalline layers of the clay. The tails must have a functional group capable of entering into a bonding interaction with the monomeric material, so that the polymers are formed between the layers of the clay.
Nevertheless, swelling with an onium ion also often proved ineffective in obtaining a proper mixing of the clay with a polymeric matrix. European patent application 0 747 322 describes that even when an onium ion with two non-polar tails is used, additional measures are necessary to homogeneously disperse a clay in a polymeric matrix which particularly consists of rubbery materials. Accordingly, the above European patent application proposes to introduce, in addition to an onium ion with two non-polar tails, one or more host molecules, also surfactant-like molecules, between the clay layers. A drawback of this method is that it is very complicated and does not in all polymeric materials enable a homogeneous dispersion of clay in the polymeric matrix.
Another approach is described in U.S. Pat. No. 5,578,672. This approach comprises two steps. In the first step, an aqueous layered clay is swollen with monomers, oligomers or polymers that are compatible with water. This leads to a partly hydrophilic material. This first step is often referred to as the so-called intercalation. The distance between the clay layers is thereby increased. The second step comprises the mixing of the intercalated clay with a second polymer. This step is referred to as the so-called exfoliation and must lead to loose individual clay plates. The resulting product is finally included in the desired polymeric matrix through extrusion. In this procedure it is essential that the clay contains a specific minimum content (usually at least 5%) of water. The method described in this U.S. patent is rather laborious and complicated. Moreover, this method is not applicable to many polymeric matrices, so that a clay cannot be included in every polymeric material.
The international patent application WO-A-93/04118 describes a composite material on the basis of a polymeric matrix and a clay, which clay is modified with a specific compound. This specific compound consists of a silane or an onium group and a group compatible with the polymeric matrix. It has turned out that with this specific compound a substantial dispersion of a clay in a polymeric matrix is only obtainable with nylon as polymeric matrix.